The following discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
NQR has been proposed as an alternative technology for the detection of explosives, narcotics and other illicit substances containing quadrupole nuclei responsive to the NQR phenomenon. Whilst NQR detection technology in theory may be used to detect explosives in items at airports, mail centres and entry points to important buildings such as courthouses etc, in practice, great difficulty has been encountered in designing and producing an NQR scanner that is practicable and reliable to operate in such environments.
An NQR scanner device theoretically brings an item to be scanned, such as luggage, to stop within a coil or near a coil or coils. The computer in control of the device directs the transmitter to irradiate the item being scanned with a burst of radiofrequency (RF) waves, which excite an NQR substance within the item being scanned. After a short period of dead time the receiver detects any very small voltage induced on the coil by relaxing quadrupolar nuclei. The computer then receives and transforms this signal and determines whether this signal exceeds a predetermined threshold level. If the signal does exceed the threshold level an alarm is signalled to the operator.
It has been found, however, that the entire process is dependent upon the physical characteristics of the item being scanned and the surrounding environment. Certain characteristics of the scan item affect the detection capability and the false alarm rate.
One of the major problems that has so far restricted the use of NQR technology is the fact that the NQR frequencies of different NQR substances drift with temperature. The amount of drift is not large (427 Hz/° C. for the 5.2 MHz line of RDX), but is enough to cause problems in measuring small hard to find signals as would be required in a practical NQR scanner. The main problem is that if the temperature of the item is unknown, then the resonant frequency of the NQR substance will also be unknown. Consequently, the item may be irradiated at the incorrect frequency resulting in a missed detection.
To overcome this problem the range of frequencies irradiated can be enlarged by using a lower transmit Q factor, however this would still result in a possible missed detection because a high receive Q factor is usually required to detect such small NQR signals.
An alternative method is to selectively irradiate small sections of the frequency spectrum sequentially so the probability of detection increases. By using this method a range of frequencies corresponding to a range of temperatures can be irradiated maximising the probability of detection. For instance, in an airport scenario for detecting an explosive having NQR nuclei, it may be expected that the range of temperatures encountered may be 0–40° C. Hence by arranging the system to irradiate all frequencies that correspond to that temperature range, the explosive should be detected.
One of the problems with the above method is the fact that it requires the use of sequential pulse sequences, each of which consume valuable time. During airport scanning of baggage, the time allowed for scanning a bag approximately ranges from 6–20 seconds. By scanning to cover a range of temperature of say 10–35° C., as many as 3 or 4 pulse sequences have to be applied so that the explosive is properly irradiated. This method results in an unacceptably long delay, which is unacceptable to the machine operator and passengers alike.
Furthermore, when irradiating the bag with two separate pulse sequences, there is a strong possibility that the explosive will be partially excited on the first sequence, resulting in a weak signal which is not detected, and then partially excited again on the second sequence, which is also not detected, resulting in a missed detection.
Another possibility is to interleave the pulse sequences such that each successive pulse group has one of the 3 or 4 frequencies required. However, the time taken to achieve the same signal sensitivity is still 3 or 4 times longer as compared to a single pulse sequence.
In the process of scanning objects and relying upon RF frequencies for signal detection, NQR scanners invariably encounter the problem of RF interference. The RF interference can be external or internal to the machine. One form of internal interference can emanate from the item being scanned. Items such as mobile phones, toys, video cameras, watches etc all emit RF noise resulting in difficulty in the detection of weak NQR RF signals. Pulse sequences that have been used to overcome magnetoacoustic ringing, may partially counteract this problem, however these techniques cannot fully remove this phenomenon.
A further problem is that the detection of illicit substances containing NQR nuclei encased within metal objects using conventional NQR techniques is difficult. The RF waves generated by an NQR scanner device are generally unable to penetrate metal surfaces due to the eddy current effect. This means that in scanning airport luggage for explosives, it may be possible to pass an explosive in a bag through an NQR scanner and it not be detected by the scanner. This problem makes current NQR scanners deficient in their capability, and is one of the principal reasons as to why no commercially viable NQR scanners have appeared in the market.